1. Field of the Invention
The present invention relates to means for detecting toner concentration in a two component type developer for an electrostatic copying machine or other apparatus which is designed to produce copies in accordance with the principle of the electrostatic photography. More particularly, the present invention pertains to toner concentration detecting means of a type which contains a differential transformer.
2. Description of the Prior Art
A so-called two component type developer for developing a latent image includes a carrier in the form of particles of a magnetic material and powders of a colored toner. In order to maintain the quality of the copies to be produced, it is required to maintain the concentration of the toner in the developer within a desired range. The U.S. Pat. No. 4,592,645 issued to K. Kanai et. al. on Jun. 3, 1986 proposes a toner concentration controlling apparatus which utilizes a differential transformer. According to the proposal by the U.S. patent, the differential transformer has a primary winding and a pair of secondary windings which are wound in opposite polarities around a core. The primary winding and one of the secondary windings provide a detection transformer which is adapted to be placed adjacent to the developer in the developer container and produces a detection signal representing the toner concentration in the developer. The primary winding and the other of the secondary windings provide a reference transformer which is adapted to provide a reference signal. The detection and reference transformers have outputs which are connected with a phase detection circuit which is adapted to detect the difference in the phase of the outputs of the detection and reference transformers. A similar toner concentration detection device is also disclosed by the Japanese laid-open patent application 60-154275 disclosed for public inspection on Aug. 13, 1985.
In using the toner concentration detecting device of this type, it is required to make a toner concentration adjustment after it has been mounted on the developer container so that any manufacturing tolerances are compensated for. Although not specifically illustrated in the aforementioned U.S. patent, described therein is an adjusting screw provided in the magnetic gap of the reference transformer for adjusting the coupling coefficient of the reference transformer.
A conventional type arrangement for the adjustment of the coupling coefficient of the reference transformer provides a variable capacitor between the primary and secondary windings.
However, the conventional arrangement has inconveniences in determining the type of variable capacitor to be used. A ceramic variable capacitor is preferable in that it is compact and has a high heat resistant property. However, it has an inherent disadvantage of a setting drift wherein the capacitance of the capacitor changes after a certain time period due to a change in the thickness of the silicon oil film which is provided between the rotor and the stator of the capacitor. Therefore, it is difficult to carry out an accurate adjustment with the use of the ceramic variable capacitor. Other types of capacitors are not preferable because they are generally bulky and do not have satisfactory heat resistance.
A further problem in using a variable capacitor is that when it is desired to locate the variable capacitor in a readily accessible position it is required to provide an extension of a high frequency wiring. Such a high frequency wiring can cause noises which may lead to an unreliable operation of the detecting device. Therefore, it is difficult to locate the variable capacitor in a readily accessible place.